The long-term goal of this work is to identify the genetic mechanisms underpinning the development and function of the visual system in a systematic and unbiased fashion. In the period preceding this grant proposal, we have carried out forward-genetic screens in search of mutations that disrupt visual functions of zebrafish. We introduced new mutations at random positions in the genome using the chemical mutagen ethylnitrosourea. Semi-automated behavioral screening assays were devised and employed to detect deficits in visual processing in over 2,000 mutagenized families. We also screened over 400 existing zebrafish mutants. This effort led to the identification of over 80 mutants. In this grant, we will now study the orderly formation of retinotectal synapses using two of the mutants discovered in our screen. We are particularly interested in the role of axon-axon interactions and activity-dependent synaptic competition in determining the exact position and territory that an individual retinal arbor is occupying in the tectum. One of our mutants lacks all retinal ganglion cells -- its tectum is genetically depleted of visual input. We will now transplant single or few wildtype ganglion cells into this mutant and ask if the position and shape of the axon arbor is altered when no other retinal axons are present. Another mutant has altered synaptic transmission properties allowing us to ask what role electrical activity plays in shaping retinotectal connections and how an activity-deficient axon arbor develops when it is surrounded by axons with normal activity levels. Finally, we propose to generate transgenic lines that allow us to conditionally block synaptic transmission from ganglion cells to neurons in the tectum. Transplantations and single-axon labelings will again serve to reveal the role of activity and synaptic competition in the establishment and stabilization of neuronal connections. Health-relatedness. Human and fish visual systems are very similar. Our approach, therefore, may lead a way to understanding the molecular basis of inherited and acquired afflictions of the visual pathway in human patients.